Geolocation processing computes the ortho-geolocation, i.e. the geodetic latitude and the geocentric longitude, both corrected from the real altitude of the Earth's surface. The (x,y) quasi-cartesian coordinates, across and along track, are also computed on the grid on which acquired radiometric data will be re-sampled at Level-1B.
Geolocation processing uses the Earth Explorer CFI functions as well as internal functions developed to deal with the geometric thermo-elastic deformations of the instrument with respect to the platform.
Two additional grids are defined during this step: the quasi-cartesian grid, and a synthetic tie-point pixel grid, defined beyond the range of pixels contained in the Earth view.
This process is composed of five main steps:
- Quasi-cartesian grid generation, enabling calculation of the (lat, lon) coordinates of the tie-points and y-coordinates of sub-satellite points necessary for further processing.
- Tie-point time calibration and scan angles, enabling calculation of the pixel number, time and angle for each synthetic tie-point pixel.
- Sentinel-3 orbit position and attitude interpolation, providing the positions on-orbit and the attitude of the satellite at each pixel time on the basis of an interpolation of the navigation and attitude information contained within the orbit file and a default attitude model embedded in the mission CFI routines.
- Instrument pixels geolocation, aiming to calculate the ortho-geolocation of instrument pixels. Solar and viewing angles are also computed for each tie-point pixel.
- Coordinates (x,y) calculation and interpolation of pixel positions, enabling derivation of the (x,y) coordinates for each instrument pixel from their (lat, lon) coordinates.